This invention relates to jet aircraft of a type having gas turbine engine exhaust effluent discharged over the upper surface of a wing or in close proximity to other aircraft structure. More particularly, this invention relates to an exhaust nozzle for a gas turbine engine wherein the temperature of the exhaust effluent is controlled to prevent overheating of aircraft structure in close proximity to the effluxing engine exhaust gases.
In certain types of aircraft, the jet propulsion engines are necessarily arranged so that the thrust producing effluent flows over, or in close proximity to, adjoining aircraft structure. For example, in one type of aircraft employing the so-called blown upper surface techniques, the gas turbine engines are mounted to discharge exhaust gases rearwardly across the upper surface of the wing to thereby increase lift. In such an arrangement, the engines are mounted such that the exhaust effluent flows in a direction substantially parallel to the normal airflow across the upper surface of the wing. Since the exhaust gases produce a greater pressure differential between the upper and lower surface of the wing than would be provided by normal airflow, the wing produces increased lift.
Such an arrangement, although advantageous from an aerodynamic viewpoint, often causes overheating of that portion of the wing surface subjected to the direct impingement of the high temperature turbine exhaust gases. Thus, considerable effort has been directed to designing the adjoining aircraft structure such that the structure will withstand high temperatures or configuring the exhaust arrangement such that overheating will not occur.
One attempt to maintain the temperature of aircraft structure that adjoins a gas turbine engine exhaust orifice within acceptable limits is disclosed in U.S. Pat. No. 3,154,267, issued to C. H. Grant. In the Grant patent, gas turbine engines are located ahead of the aircraft wing and discharge turbine exhaust gases rearwardly across the upper surface of the wing. An air inlet opening coaxially surrounds each engine to duct atmospheric air to the vicinity of the engine tail pipe. The atmospheric air flowing through the air inlet effectively surrounds or blankets the hot exhaust stream prior to impingement of the exhaust stream on the wing surface. Additionally, in some embodiments of the arrangement disclosed by Grant, an air passage having an inlet opening positioned forward of and below the leading edge of the wing directs a sheath of atmospheric air across the portion of the upper surface of the wing that is in proximity to the engine exhaust orifice. This sheath of atmospheric air effectively forms a cool boundary layer or blanket of air between the hot turbine exhaust stream and the wing surface.
Although an arrangement such as that disclosed by Grant can be effective in controlling the temperature of the airfoil surface, such an arrangement requires additional aircraft structure in that additional air passages must be provided and the gas turbine engines must be mounted in a particular orientation. Further, the arrangement disclosed by Grant does not provide the most efficient use of the cooler air to produce maximum engine thrust. In this regard, it is known that the forced mixing of a relatively low velocity, low temperature secondary gaseous flow with the relatively high velocity, high temperature turbine exhaust gases prior to discharging the engine effluent into the atmosphere produces both increased thrust and a lower noise level than are achieved by exhausting the turbine exhaust gases alone. For example, in modern gas turbine engines of the turbofan variety, a portion of the fan air is commonly ducted around the engine compressor, combustor, and turbine stages and is mixed with the turbine exhaust gases either in a mixer stage that is internal to the engine, or in an exhaust nozzle mounted on the rear portion of the engine.
Such mixing arrangements generally include a tubular mixer section coaxially mounted within an airflow duct with the mixer section often being coaxially mounted around an axially extending engine plug. Fan air is introduced into the annular duct formed between the exterior surface of the mixer section and the inner surface of the airflow duct and the turbine exhaust gases are introduced into the annular duct formed between the interior surface of the mixer section and the outer surface of the engine plug. The high temperature, high velocity turbine exhaust mixes with the lower temperature, lower velocity fan air as the two fluid streams flow past the mixer section exit plane.
One type of the above-described mixing apparatus, commonly referred to as a daisy mixer, includes a mixer section having a plurality of axially extending circumferentially spaced lobes or corrugations of increasing radial dimension relative to the axial flow of exhaust gases through the mixer section. The corrugations or lobes "force" mixing of the fluid streams by increasing the peripheral length at the mixer section exit opening to greatly increase the boundary region between the two flow streams and hence cause thorough mixing of the turbine exhaust gases and the fan exhaust gases.
Forced mixing results in increased thrust since the mixed gases have a higher mass-velocity product than the mass velocity product of the turbine exhaust gases alone. Further, since the component of noise produced by the pressure disturbance that is created by the discharged exhaust stream is proportional to the mass velocity of the discharged exhaust stream exponentially raised to a high power (typically 8), the noise produced by an engine including forced mixing is substantially lower than that noise produced by an engine directly exhausting the high velocity turbine gases.
Although such prior art mixed flow arrangements are advantageous in that significantly more thrust is produced and lower noise levels are obtained, prior art forced mixers do not reduce the temperature of the exhausted gases to the point that the mixed exhaust stream can impinge on, or be discharged in close proximity to, conventional aircraft structure. Thus, in relation to upper blown surface arrangements, or other arrangements in which the engine exhaust gases impinge on aircraft structure, the artisan has been faced with the choice of foregoing the significant advantages of forced mixing or using additional means in combination with a mixed flow exhaust to protect the aircraft structure. Such additional means can include special design of the affected aircraft structure, e.g., multiple layers of covering material with or without an internal cooling system, or can include the introduction of additional cooling air that is not forcibly mixed with the exhaust effluent to thereby form a protective boundary layer as is disclosed in the Grant patent. Each of these alternatives complicate the aircraft structure and impose undesirable weight penalties.
Accordingly, it is an object of this invention to provide mixing apparatus for use in engine installations in which exhaust gases are directed across, or in close proximity to, a portion of the aircraft structure wherein the temperature of the adjoining aircraft structure is maintained within an acceptable temperature range.
It is another object of this invention to provide an exhaust arrangement for a gas turbine engine mounted in proximity to other aircraft structure wherein the exhaust effluent does not overheat the adjoining aircraft structure and the engine turbine exhaust is mixed with a cooler gaseous flow to increase the engine thrust and decrease the engine noise level.
It is yet another object of this invention to provide a multilobed forced mixer for use in gas turbine engines that direct engine exhaust gases across adjoining aircraft structure wherein the temperature of the adjoining structure is maintained within acceptable limits.
It is still another object of this invention to provide gas turbine exhaust arrangements supplying the above objectives without increasing the weight or complexity of either the gas turbine engine or the adjoining aircraft structure.